Digital protection devices are used in power systems to protect electrical generators and other system components, such as transformers and transmission lines, using voltage and current waveforems. Such digital protection devices may be embodied in a generator protection unit (GPU), protective relay, or the like. For example, a typical GPU estimates voltage and current phasors that are determined from the "fundamental" frequency of the system, which is based upon voltage waveforms, and from such phasors detects various fault conditions that may require a circuit breaker to be tripped. The phasors are also used for metering and other power system control functions. Abnormal conditions may also be detected through relatively large changes in the generator's operating of fundamental frequency. Accordingly, a technique for frequency tracking is important for both deriving accurate voltage and current phasors and for monitoring fault conditions in the generator by detecting a significant change in frequency. Further background information concerning the present invention may be found in U.S. Pat. No. 5,832,413, Nov. 3, 1998, "Generator Protection System and Method for Phasor Estimation and Frequency Tracking During Frequency Ramping," the disclosure of which is hereby incorporated by reference in its entirety.
In typical protection systems of the type discussed above, frequency is calculated using only measured system voltages. There are scenarios, however, in which the voltages are not available and hence the frequency cannot be calculated. This in turn prevents the calculation of many system-wide values needed for protection and metering.